When I was at university in the late 80s, the influence of The Madwoman in the Attic, Sandra Gilbert and Susan Gubar's milestone feminist study of the Victorian imagination, could be felt in every corner of the department of English: liberating breeze or poisonous miasma, depending on your point of view. As a result, our studies were onerous. It wasn't enough to read Jane Eyre, Villette and Middlemarch. Beside them on your desk would be a teetering stack of books about Victorian attitudes to sex and science, to marriage and mesmerism, to geology and phrenology and the female malady – plus a pile of minor novels, too. The sensational and monstrously swollen stories of Mrs Henry Wood and Mary Braddon were, we were told, ripe for reconsideration. At the end of a week of this, exhaustion and terror combining to bring me halfway to a swoon, I felt like the heroine of one of these febrile tales myself: dazed, confused and in desperate need of, if not smelling salts, then at least a Twix.

Future students are going to have it easier. They will need only to turn to Mrs Robinson's Disgrace. Yes, on the surface of it, Kate Summerscale's new book is a straightforward account of misplaced love and misguided betrayal. Like her award-winning The Suspicions of Mr Whicher, it blows the dust off long-forgotten people and events, and sees in them the seeds of literary inspiration. If Mr Whicher was a model for writers of detective stories from Wilkie Collins on, Mrs Robinson is a real-life Lady Isabel Carlyle, the sexually obsessed wife in Mrs Henry Wood's 1861 bestseller, East Lynne. (She also resembles, as her publisher trumpets, Emma Bovary, though she surely wasn't that character's inspiration: the dates don't tally.) But Mrs Robinson's Disgrace is also a vast section of Victorian thought in microcosm, a breathtaking achievement its author pulls off almost casually in (discounting her extensive notes and bibliography) 226 scant pages.

You would not believe the walk-on parts her narrative turns up. Dickens strolls by, and Darwin, while the novelist Catherine Crowe is at one point found wandering the streets of Edinburgh, mad and stark-naked. Summerscale's heroine, Isabella Robinson, is an acquaintance of the publisher Robert Chambers, whose bestselling Vestiges of the Natural History of Creation was a daring, proto-evolutionary account of the Earth's formation – and she was a busy correspondent of George Combe, the phrenologist whose fame was such that Queen Victoria invited him to examine the head of her nine-year-old son, the Prince of Wales.

Isabella's putative lover, Edward Lane, is the proprietor of a fashionable spa, Moor Park, at which patients tried the new fad for hydropathy, and his brother-in-law, George Drysdale, is the author of Physical, Sexual and Natural Religion, a guide to contraception and sexual disease, which argued that desire was natural and masturbation no substitute for the real thing, since it kept the natural passions "pent up in the gloomy caverns of the mind". Against this enlightened backdrop, her infatuation with Lane, the cause of her subsequent "disgrace", seems almost mundane – until, that is, you remember that beyond her circle the world was still corseted in fear and loathing. In 1850 a woman who desired to have intercourse with a man other than her husband was usually classified as suffering from sexual mania, a form of madness. For Isabella this must have been bewildering: the pull of free thought and the push of repression. It would, I think, have crushed a lesser woman altogether.

The story begins in 1844, when Isabella, a widow from a good family, married Henry Robinson, a civil engineer, on his third proposal. "I suffered my scruples & dislike to be talked away by others," she later explained, "& with my eyes almost open I walked into the bonds of a dreaded wedlock like one fated." The marriage was advantageous to them both: as the mother of a small child, Isabella could not afford to be picky, while for Henry it brought status and money. For the sake of a quiet life she signed over to him a settlement, made on her by her father, of £5,000.

Soon after, the couple moved to Edinburgh, and it was there that Isabella met Edward Lane, a "fascinating" young man who was training to be a doctor. Lane was 10 years her junior and in possession of a devoted wife, Mary, but he was a charming and able conversationalist, and he and Isabella, an intelligent and curious woman for all that she seems also to have suffered from chronic ennui, sometimes talked together about new ideas. In particular they discussed the loss of their religious faith. "I said that the grandeur of truth made up to me for relinquished hopes," she wrote in her diary afterwards.

Isabella quickly became infatuated with Lane, a passion she detailed feverishly in the pages of her diary. When Edward was responsive – or at least kind enough occasionally to answer her letters – she was on top of the world. When he was cold – when, or so the reader senses, he felt mildly alarmed by her attentions – she would lapse into headaches, listlessness and depression. The pity of it is that it wasn't until four years later (and after she had enjoyed similarly unreciprocated, if less ardent, infatuations with two of her sons' tutors) that anything of substance happened between them. In 1854 at Moor Park, Surrey, where Lane now offered patients the "water cure" (Darwin was treated for his dyspepsia with a douche aimed at his abdomen), the couple finally kissed. Isabella was in ecstasy. Was the relationship consummated? There is no evidence that it was. In her diaries she talks of her "half-realised" bliss in Edward's arms; her tone is one of sexual frustration rather than satisfaction.

But this is, and was, a moot point. In 1855, after her relationship with Lane had again cooled, Isabella was taken ill, probably with diphtheria. Looking into her room one day, Henry heard his delirious wife mutter the names of several men. He went to her desk, opened her diary and learned both of her loathing for him (not that he was exactly a devoted husband; he had a mistress and two illegitimate daughters) and of her infatuation with Lane. As soon as she was well again he told her he intended to sue for divorce.

Summerscale devotes the second half of her book to this decision and its consequences for all involved. I won't reveal who won, or how, but it is riveting to watch the new Court of Divorce and Matrimonial Cases and the newspapers which followed the case so avidly wrestling with the only serious piece of evidence available to them: Isabella's purloined diary. Was it a true account of events? Or was it the work of a fantasist? And if it was true, wasn't it a clear sign of sexual mania? Surely only a madwoman would put such shameful thoughts in writing? Summerscale's account of this court case is faultless; her seemingly inexhaustible capacity for research renders what could be tedious and dry vividly alive. She will tell you how a judge liked to wave his lorgnette, as well as what he said in court. But she deals in fact, not supposition, so when the narrative lags, or when gaps inevitably appear, she uses reading rather than imagination to enliven proceedings: the miniature essays she drops into the text about diary writing and its place in Victorian culture; about hydropathy, alienism and so many other things besides, are lively, thoughtful, and remind you how truly peculiar the Victorians could be.

There were times, I'll admit, when I longed for her to take sides, to rail against the supposedly liberal men who, on discovering that they too appeared in Mrs Robinson's diary, worried for their own reputations without ever losing a moment's sleep over hers. She is sometimes, for my taste, a little too careful, too precise, too cool. And I would have liked an afterword, telling us how she stumbled on this curious story in the first place. But mostly I'm all admiration: she has turned a sepia photograph, curling and tattered, into a film that runs through the mind in glorious and unimpeachable Technicolor.